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Christmas Moon

StarDate:

December 25, 2015

The just-past-full Moon soars high across the sky on this Christmas night, casting a silvery glow across the landscape.

The view will lose a bit of its dazzle as the night rolls by, though. For one thing, the Moon is beginning to “wane” — the fraction of its surface illuminated by sunlight is getting smaller, so there’s less area to reflect light toward Earth.

Another is the changing angle between Sun, Earth, and Moon.

When the Moon is full, it lines up opposite the Sun in our sky. The sunlight reflects directly back toward the Sun, but with Earth in the way, we catch a lot of that light.

As the Moon moves away from that alignment, though, we catch less and less of the moonlight that shines out into space. Because the lunar surface is bumpy and mottled, it’s not a perfect reflector. So as our viewing angle changes, more of the reflected sunlight is scattered away from Earth.

What’s more, at lower viewing angles, we see more shadows cast by mountains and craters, so less of the daytime portion of the lunar disk is actually lit up by the Sun.

The combination means that the waning Moon fades in a hurry. In less than three days, it’s only half as bright as when it was full — even though 90 percent of the disk is still bathed in sunlight. And when the Moon is at one of its quarter phases — when it’s really half full — it shines less than one-tenth as bright as when it was full.

So enjoy the bright moonlight — before it fades away over the coming nights.

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